Conventionally, there have been actively proposed apparatuses and methods for changing and controlling damping forces of shock absorbers disposed between a vehicle body and wheels. For example, Japanese Patent Application Laid-Open (kokai) No. 2007-8373 discloses a suspension characteristic computation method which provides a guideline for designing suspensions in consideration of a correlation between roll and pitch generated in the vehicle body. In this suspension characteristic computation method, a vertical force on a front-wheel side is computed from the product of a geometry proportionality coefficient on the front-wheel side and the square of a tire lateral force, and a vertical force on a rear-wheel side is computed from the product of a geometry proportionality coefficient on the rear-wheel side and the square of a tire lateral force. The sum of the two vertical forces is computed as a pitch moment attributable to the geometry of suspensions. Meanwhile, a pitch moment attributable to damping forces of the suspensions is computed from the product of a damping force proportionality coefficient and a roll rate. Subsequently, a pitch angle is computed from the product of the sum of the computed two pitch moments and the gain and phase delay of the pitch angle in relation to the pitch moment. The phase difference between the pitch angle and the roll angle is then computed from the computed pitch angle.
In the case where suspensions are designed in accordance with such a suspension characteristic computation method, timings at which a roll and a pitch are generated, respectively, can be synchronized, by means of, for example, properly setting an extension difference or a contraction difference between shock absorbers provided on the front-wheel side and those provided on the rear-wheel side. As a result, maneuvering stability can be improved.
Further, Japanese Patent Application Laid-Open (kokai) No. H6-99714 discloses a vehicular suspension apparatus which can perform active roll suppression control in accordance with the roll direction of the vehicle body by use of only a steering sensor. In this vehicular suspension apparatus, when a steering angle detected by a steering sensor exceeds a predetermined neutral threshold value, the control state of shock absorbers is switched to a roll control state in which left and right shock absorbers are controlled to produce larger damping forces in the expansion or contraction direction in accordance with the roll direction of the vehicle body determined from the sign of a steering angular speed. Subsequently, when the sign of the steering angular speed is reversed due to reverse steering, the left and right shock absorbers are controlled such that the shock absorbers produce larger damping forces in directions opposite the directions in which they have produced larger damping forces in the above-described roll control state.
Furthermore, Japanese Patent Application Laid-Open (kokai) No. H6-48147 discloses a vehicular suspension apparatus which suppresses rolling due to abrupt steering and prevents riding quality from deteriorating at the time of steering. In this vehicular suspension apparatus, a control signal is calculated from a bounce rate based on sprung-portion ascending/descending speed, a pitch rate detected from a difference in sprung-portion ascending/descending speed between the front and rear sides of the vehicle body, and a roll rate detected from a difference in sprung-portion ascending/descending speed between the left and right sides of the vehicle body. When the control signal is equal to or greater than a predetermined large threshold, the damping forces of shock absorbers on the expansion side (on the side toward a steering direction) are increased, and the damping forces of shock absorbers on the contraction side (on the side opposite the steering direction) are decreased. When the control signal is equal to or less than a predetermined small threshold, the damping forces of shock absorbers on the expansion side are decreased, and the damping forces of shock absorbers on the contraction side are increased.
Furthermore, Japanese Patent Application Laid-Open (kokai) No. H11-245641 discloses a roll control apparatus for a vehicle which prevents riding quality from deteriorating during high speed turning because of roll control. This roll control apparatus detects a lateral gravitational acceleration which acts on the vehicle during traveling, and switches and controls shock absorbers to produce damping forces in a direction for suppressing rolling of the vehicle, when the lateral gravitational acceleration exceeds a previously set threshold value.